414
Macromolecules 1992,25, 414-421
Thermal Analysis and Model of Ultrahigh Molecular Weight Polyethylene Gels H. Phuong-Nguyenand G. h l m a s ' Chemistry Department, Universitk du Qukbec B Montrkal, CP 8888 SUCA, Montreal, H3C 3P8 Canada Received December 26,1989; Revised Manuscript Received May 20, 1991
ABSTRACT The traces of dissolution in decalin of nascent polymer and of gels equilibrated in different conditionsare obtained at a low rate of heating (v = 1Klh). The dissolution trace showe a gap between the low- and high-temperature endotherms (fractions I + I1 and fraction 111, respectively). The equilibrium temperature of the end of diesolution depends on solvent and thermal history and can be as high as 165 O C . The heat flow during a slow crystallization reveals that two fractions grow separately in timeltemperature. Since similar patterns were found in the melting and cooling traces of the solid, these results are interpreted along the same lines although other explanations may develop in the future. The high-temperature signals correspondto melting or crystallizationunder strain. A model for the ultrahigh molecular weight polyethylene gel follows readily from thie interpretation: A strained melt network is stable in the solution at high temperature, and ita cocrystallization with chain-folded crystals is at the origin of gel formation on quenching. The heat-resistant crystals take away from entanglements the key role in gel formation. A calorimetric characterization of crystallization as gel or single cryetals is given. Procedures to reduce or raise the amount of network crystals are presented. Confirmation of the model by literature results and other techniques is discussed.
Introduction Solutions of polyethylene (PE) of low or medium molecular weight have been prepared to grow single crystals on cooling. The thermodynamics of phase change of lamellae of different thickness, 1, crystallized at different temperature, T,,has been established.' Their melting points, T m , or dissolution temperatures, T d , correlate with 1 and allow the determination of Tm,o and Td,o, the values for infinite thickness (or extended chains). Tm,o is 141 "C, and Td,o is 99 OC in decalin and 111 OC in p-xylene for instance.l T d is lower (by 5-12 K) for thin lamellae. Solutions of high or ultrahigh molecular weight PE have also been made to grow crystals, but crystals with a different morphology, the shish kebabs. These complex crystals are formed at high temperature (above Td,o) in solutions under shear. The crystals which grow on a stirrer or in the gap of a Couette viscosimeter can form fibers with an unexpectedly high drawability. A large body of research has focused on the morphology of fibers, their thermal properties, and the relationship between drawability and mode of preparation.2* The high modulus of extensively drawn fibers has given a high profile to the method of preparation involving a solution rather than a melt. Shish kebabs are easily superheatable due to their slow Drawn fibers7have an equilibrium melting temperature above T,,o when they are submitted to strain during melting, the normal entropy of melting being reduced by the constraints. Dissolution behavior of shish kebabs, dried gels, and fibers indicates that they are also superheatable in the presence of a solvent. No equilibrium dissolution temperature has been measured in a calorimeter due to the lack of an adequate apparatus. However, from measurementa on drawn fibers in p-xylene, Torfs et al. arrived at the conclusion that without external strain the equilibrium dissolution temperature T d is very near Td.o. Measurement of T d was carried out visually or by measurement of the temperature of disappearance of stress for strained fibers. On cooling, solutions form gels which can be drawn in a dry or wet state into fibers. The occurrence of gels from
solutions of an easily crystallized polymer and other unusual features of solutions of high molecular weight PE (such as the effect of thermal and mechanical history on the state at room temperature) raised questions about the underlying association of macromolecules in a dilute solution?JO The network structure recognized in fibers and nascent polyme#J1J2 is thought to be at the origin of the gel cohesion, entanglements forming the stable junctions. In order to explain the long-time memory effects in melts and in noncrystalline solutions of long-chain macromolecules, especially the stable entanglements, the existence of tight knots was proposed. They could be generated during crystallization or in a sheared s01ution.l~ Crystalline junctions induced by shear were nevertheless postulated9 although there was no support of an equilibrium melting endotherm at high temperature to give a final proof of their existence. In recent a missing part in the heat of fusion of nascent ultrahigh molecular weight polyethylene (UHMWPE) was found using a slow heating rate (u = 1 K/h) instead of the usual for the differential scanning calorimetry (DSC) analysis (600 K/h). It was called fraction 111,fraction I being constituted by the crystals with a low melting temperature and fraction 11by the main melting peak of unstrainedchain-foldedor extended-chain crystals. The strained crystals of fraction I11are very stable in time/ temperature; their melting does not occur below 200 OC if u is higher than 12 K/h. On cooling, the strained melted chains recrystallize at high temperature as a network in the melt. The stability of fraction 111 in the presence of a solvent is the object of this paper. Its importance is due to the fact that it affects the properties of dilute solutions and of gels and also of dry gels. The conditions for maximum drawability which depend on the solution preparation (Le., solvent, concentration, and maximum temperature reached) are still unclear. In this laboratory, investigation of the stability of fraction I11 has been made by intrinsic viscosity and calorimetry in a number of solvents (decalin, trichlorobenzene, normal and branched alkanes, and aromatic solvents) chosen for their solvent quality or their recognized advantage for characterization purposes (tri-
0024-929719212225-0414$03.00/0 0 1992 American Chemical Society
Macromolecules, Vol. 25, No. 1, 1992
Thermal Analysis a n d Model of Polyethylene Gels 415
Table I Characteristics of Dissolution of Nascent (A, B) and Gels (C-G) in Decalin (0.2%) of PE GUR Having Different Thermal Histories
A
B C
D E F G
thermal history nascente annealedb,e sameasBf
g
sameasB 4 days at 150 "C sameasB
n 1 1 >2 7 3 2 3
residence time" >3days 9 months 7 months 2 days 6 months
u,
Kih 1 1 1 1 1 1
6
Td,"C 109.0 109.0 96.0 97.6 98.0 97.4 96.7
fractions I + I1 H,'Jig 180 (105-114) 180 (102-113) 260 (90-102) 160 (92-100) 235 (92-101) 230 (90-101) 210 (92-102)
fraction I11 H3.atdJig &.b, Jig 50 (128-144) 58 (145-165: 88 (131-142) 20 50 (100-120) 90 (130-158) 16 (119-127) 34 (127-141) 60 (106-126) 60 (108-198)
figure 1 2 3a 3b 4a
At room temperature, between the last crystallization and the dissolution. One week at 90 "C in decalin (below Td). Interval of melting is noted in parentheses. H3,b is included in H3,8when the two endotherms overlap. e First dissolution. f Typical dissolution trace after a network weakening treatment and a short residence time at room temperature. g From the solution of nascent GUR which waa submitted to several cycles of dissolution-crystallization; T,, = 120 "C. @
Table I1 Characteristics of Crystallization and Dissolution of Single Crystals and Mixed Morphologies of PE GUR in Decalin (0.2%) crystallization dissolution v, fractions I + I1 fraction I11 v, fractions I + I1 fraction I11 n Kih T,,""C H,Jig H3,bJig figure n Kih TdF°C H, Jig H3,b Jig figure H 3 6 80.3 (93) 230 62(123-160) 4b 4 6 90.5-96.4 220+ 35 40(115-185) 4~ I 3 1 83.0 (88) 267 15 (88-92) 5a 4 1 92.4-96.6 207+ 55
